There are a lot of opportunities to reproduce audio signals at a noisy place, where a noise level is high and varies, such as a factory, and transportation means including cars and trains.
FIG. 6 is a block diagram illustrating a conventional audio reproducing apparatus that is utilized in a noisy environment. In FIG. 6, an audio signal received at input terminal 1 travels through variable gain circuit 2 and is amplified by power amplifier 3. An output signal tapped off from amplifier 3 is fed into speaker unit 5 disposed in speaker box 4, thereby reproducing the signal into audio form.
Microphone 6 disposed around speaker unit 5 collects the sum of the signals which speaker unit 5 radiates and noises around speaker box 4. An output signal supplied from microphone 6 together with the output signal from amplifier 3 are fed into subtractor 7.
Subtractor 7 subtracts an input signal component from the sum of the signal radiated from speaker unit 5 and the noises around speaker box 4 so that an output signal proportional to the surrounding noise is extracted. The output signal supplied from subtractor 7 runs through low pass filter 8 where a frequency band of the signal is limited, and then runs through rectifier 9 where the output signal is converted into direct current (dc), which then is fed into variable gain circuit 2.
Because variable gain circuit 2 is automatically controlled its amplifying rate by the output signal supplied from subtractor 7, speaker unit 5 can radiate signals free from being cancelled by the surrounding noise, i.e. free from being masked. As already discussed, the output signal supplied from subtractor 7 varies proportionally to the surrounding noise, which contributes to this automatic controlling.
However, in the conventional audio reproducing apparatus utilized in the noisy environment, differences exist between the audio signals supplied from speaker unit 5 and the output signals supplied from amplifier 3 so that subtractor 7 cannot completely remove the radiated signal from speaker unit 5. Therefore, it is difficult to extract only the noises around speaker box 4, and a user is thus obliged to control variable gain circuit 2 within a narrowly limited frequency band. The conventional audio reproducing apparatus utilized in a noisy environment thus has not been provided with a sufficient masking correction.